Uninterpretablity in depth maps: why the pixel values do not necessarily represent a valid distance?
As @troy_s suggested, PNGs are not suitable for this use. PNGs are more suited to image color data only. For depth information you should use a format such as Open EXR. This can be achieved easily by simply changing the depthOutputNode.format.file_format = 'PNG' line to depthOutputNode.format.file_format = 'OPEN_EXR'.

The resultant image should now contain the correct depth information. For example, it can be loaded into the compositor, normalized, and passed to a Viewer node as shown :